It's in the agreement for Class Two users of the FAA-provided source of filtered data called ASDI. It's simply a restriction the FAA has imposed on this data that they provide.

It doesn't, for example, forbid you to use such data if you are otherwise able to acquire it. This is why sites like FlightRadar24 and whatnot can use volunteer-based ADS-B feeders to display truly "live" data that augments their fallback source of the 5-minute delayed data provided by the FAA.

Thanks SWA4678 for the info on the flight delay. The entire Agreement Memorandum for class two users(i.e. FlightAware and others) can be read by going to the fly.faa.gov site, or type in your browser "asdi class two". Its a PDF file. Make sure you look at the Ver 1.4A file. The 5 minute or more delay information is on page 2 of the 13 page agreement.